The disease manifests as small circular spots measuring 2–5 mm, light brown or grayish in color with a characteristic purple or reddish-brown border. Under high humidity, a velvety coating, representing the fungus's sporulation, forms in the center of the affected areas. Subsequently, the spots coalesce, causing yellowing, necrosis, and premature death of the leaf blade, which forces the plant to expend stored nutrients on foliage regeneration.
The development of the pathogen depends on weather conditions, requiring temperatures in the range of +20 °C to +30 °C and a high relative humidity above 70–90%. The presence of free moisture on the leaf surface is critical for the fungus to penetrate through the stomata. The infection persists in the soil on plant debris as mycelium and can be transmitted via seed material.
Evaluation of the resistance of varieties and hybrids is conducted during variety trials based on a visual method, where the degree of infestation is recorded as a percentage of the leaf area or on multi-point scales. Experts use phytomonitoring and regular surveys to determine the infection spread dynamics. Based on the observation results, samples are ranked by resistance groups, which helps agronomists select hybrids capable of maintaining a healthy leaf apparatus until the end of the vegetation period.